The present invention relates to an oscillation gyro which is mounted on an automobile or the like for detecting an angular velocity caused by steering or the like, and which is used for an attitude control system or a navigation system of a vehicle.
FIG. 25 is a construction diagram of an oscillation gyro which is described in Japanese Application No. 28140/1993 which has previously been filed by the inventor. In FIG. 25, numeral 1 designates an oscillator having a regular quadrangle shape, which is composed of a constant elasticity material of elinvar or the like. Numerals 2 and 3 designate piezoelectric units for driving the oscillator 1, which are fixed to two contiguous column faces of the oscillator 1 by using a conductive adhesive agent or the like. The oscillator 1 and the piezoelectric units 2 and 3 constitute an oscillating unit. Numerals 4 and 5 designate current-voltage converters as current-voltage converting means, which are respectively connected with noncommon electrodes of the piezoelectric units 2 and 3. Numeral 6 designates an adder for adding outputs of the current-voltage converters 4 and 5, and numeral 7 designates an amplifier for oscillating the oscillator 1 by a self-excited oscillation, by amplifying an output of the adder 6 and by feeding back the amplified signal to the oscillator 1 which also operates as a common electrode of the piezoelectric units 2 and 3, which functions as a piezoelectric unit driving means for driving the piezoelectric units 2 and 3. Further, at the output side of the adder 6, an amplitude detecting means 8 for detecting an amplitude of the output from the adder 6, and a controller 9 for controlling a gain of the amplifier 7 by receiving an output of the amplitude detecting means 8. The amplitude detector 8 and the controller 9 control the amplitude of the output of the adder 6 to be a predetermined value. An output of the amplifier 7 is also inputted to an inverting amplifier 7 having a gain of K, of which output is inputted to the current-voltage converters 4 and 5 through condensers 11a and 11b, each having a capacitance of Cr. The inverting amplifier 10 and the condensers 11a and 11b constitute an eliminating means for eliminating damping capacitances of the piezoelectric units, mentioned later, by amplifying the output of the amplifier 7, shifting the phase of the output by approximately 90.degree., and supplying the output to the current-voltage converters 4 and 5. Numeral 12 designates a differential amplifier as an error calculating means for calculating a difference between the outputs of the current-voltage converters 4 and 5, which outputs a signal corresponding to the angular velocity.
An explanation will be given of the operation of the conventional oscillation gyro shown in FIG. 25. A voltage V is supplied from the amplifier 7 to the oscillator 1 which operates as a common electrode of the piezoelectric units 2 and 3. On the other hand, the noncommon electrodes of the piezoelectric units 2 and 3 are connected to inverting input terminals of operational amplifiers 41 and 51 respectively composing the current-voltage converters. Further, the non-inverting input terminals of the operational amplifiers 41 and 51 are grounded. Accordingly, the noncommon terminals of the piezoelectric units 2 and 3 are respectively in an imaginarily-grounded state whereby the piezoelectric units 2 and 3 drive the oscillator 1 in a predetermined driving-axis direction by the voltage from the amplifier 7. Currents from the noncommon electrodes of the piezoelectric units 2 and 3 respectively flow into the inverting input terminals of the operational amplifiers 41 and 51, as shown. Further, currents from the condensers 11a and 11b also flow into the inverting terminals of the operational amplifiers 41 and 51. Therefore, the output voltages of the operational amplifiers 41 and 51 are determined by a current value added with each of the currents flowing in the piezoelectric units 2 and 3 and each of the currents flowing in the condensers 11a and 11b, respectively, and resistance values for respective feedback resistors 42 and 52.
An explanation will be given of the operation of the current-voltage converters 4 and 5 based on FIG. 26. As shown in FIG. 26, the piezoelectric unit 2 is expressed by an equivalent circuit composed of L(21), C(22), R(23) and Cd(24). A current I3 flowing through the condenser 11b is given by the following equation. EQU I3=j.omega..times.Cr.times.K.times.V (1)
On the other hand, a current I2 flowing in the Cd(24) which is the damping capacitance of the piezoelectric unit 2 is given by the following equation . EQU I2=j.omega..times.Cd.times.V (2)
Under this state, when K=Cd/Cr, the I2 flowing into the inverting input terminal of the operational amplifier 41 is equal to the I3 flowing out of the inverting input terminal, and therefore, only a current I1 flowing in a series resonance circuit composed of L(21), C(22) and R(23) flows into the inverting input terminal. In this way, the current I1 is equal to a current I4 flowing in a resistor 42 connected to the operational amplifier 41, and therefore, the output of the operational amplifier 41 corresponds to the current flowing in the series resonance circuit.
Further, although the above explanation has been given of the piezoelectric unit 2 and the current-voltage converter 4, the same explanation is also applicable to the piezoelectric unit 3 and the current-voltage converter 5.
The outputs of the current-voltage converters 4 and 5 are signals corresponding to the series resonance components of the equivalent circuit of the piezoelectric units 2 and 3, and the series resonance components are determined by the mechanical resonance characteristic of the oscillator 1 and the force factor of the piezoelectric units. Further, the adder 6 receives the outputs of the current-voltage converters 4 and 5, and outputs the sum of both. Accordingly, the output of the adder 6 is also determined by the mechanical resonance characteristic of the oscillator 1 and the force factor of the piezoelectric units. When such a signal is fed back to the oscillator 1, the oscillator 1 is oscillated in a self-excited oscillation at a mechanical resonance point.
As stated above, the oscillation gyro of FIG. 25 oscillates the oscillator 1 in a self-excited oscillation by feeding back the signal which is-determined by the mechanical resonance characteristic and the force factor of the piezoelectric units, and detects the angular velocity by detecting a difference between the currents flowing in the series resonance components in the equivalent circuit of the piezoelectric units 2 and 3 and by amplifying it.
Next, an explanation will be given of the principle of detecting the angular velocity by a vibration gyro.
FIG. 27a is a symbolic expression of the piezoelectric units 2 and 3, showing that, when the same potential V is applied on the two piezoelectric units, currents of IL and IR respectively flow therein. As shown in FIG. 27b, when the voltage V is applied on the piezoelectric units 2 and 3, the piezoelectric units 2 and 3 generate FL (a force by the piezoelectric unit 2) and FR (a force by the piezoelectric unit 3) in the orthogonal directions of the piezoelectric units. These forces FL and FR are expressed by the following equations by using unit vectors i and j in the detecting-axis direction (transverse direction) and in the driving-axis direction (longitudinal direction). A in the following equations indicates the force factor of the piezoelectric unit. EQU FL=A.times.V.times.cos 45.degree.i.times.+A.times.V.times.sin 45.degree..times.j (3) EQU FR=-A.times.V.times.cos 45.degree..times.+A.times.V.times.sin 45.degree..times.j (4)
By synthesizing these two forces FL and FR, the oscillator 1 receives a force only in the driving-axis direction, and moves in the driving-axis direction at a velocity of vY.
Under this state, the oscillator 1 is applied with an angular velocity .OMEGA. by steering of a vehicle or the like as shown in FIG. 27c. At this moment, a Coriolis force FC is generated in the oscillator 1 in the detecting-axis direction. The size of FC is given by the following equation by defining an equivalent mass of the oscillator 1 as m. EQU FC=2.times.m.times..OMEGA..times.vY.times.i (5)
The oscillator 1 oscillates in the detecting-axis direction and the driving-axis direction respectively with velocities of vX and vY by the forces FL and FR which are generated by the piezoelectric units 2 and 3 and the Coriolis force FC, and a reactive force FZ is generated. The size of the reactive force is given by the following equation by defining a mechanical impedance in the detecting-axis direction as ZX, a mechanical impedance in the driving-axis direction as ZY. EQU FZ=-ZX.times.vX.times.i-ZY.times.vY.times.j (6)
The following equation is established since the forces FL and FR generated by the piezoelectric units 2 and 3, the Coriolis force FC and the reactive force FZ are balanced. EQU 0=FL+FR+FC+FZ (7)
Therefore, the following equations are obtained by substituting the equations (3) through (6) to the equation (7), and by separating it with respect to the "i" term (in the detecting-axis direction) and the "j" term (in the driving-axis direction).
In the detecting-axis direction: EQU 0=2.times.m.times..OMEGA..times.vY-ZX.times.vX (8)
In the driving-axis direction: EQU 0=2.times.A.times.V.times.sin 45.degree.-ZY.times.vY (9)
The angular velocity .OMEGA. is provided from these equations of (8) and (9), as follows. EQU .OMEGA.=(ZX.times.ZY.times.vX)/(4.times.m.times.A.times.V.times.sin 45.degree.) (10)
On the other hand, the currents IL and IR flowing in the piezoelectric units 2 and 3 are expressed by the following equations by defining the damping admittance of the piezoelectric units 2 and 3 as Y, and defining the oscillation velocities in the respective orthogonal directions as VL and VR. EQU IL=A.times.vL+Y.times.V (11) EQU IR=A.times.vR+Y.times.V (12)
Further, VL, VR, vX and vY are in a relationship of FIG. 27d in view of a vector relationship.
Hence, EQU vX=cos 45.degree..times.(vL-vR) (13) EQU vY=sin 45.degree..times.(vL+vR) (14)
At this point, the equation (11) is subtracted by the equation (12), and (vL-vR) is eliminated by the equation (13), as follows. EQU (IL-IR)=A.times.(vL-vR)=A.times.vX/cos 45.degree. (15)
By using the equation (15), vX in the equation (10) is eliminated to provide the equation (16) as follows. ##EQU1##
Further, the following equation is given by adding the equations (11) and (12). ##EQU2## vY is eliminated by using the equation (9) in the equation (17), as follows. EQU (IL+IR-2.times.Y.times.V)=2.times.A.sup.2 .times.V/ZY (18)
By substituting the equation (18) to the equation (16), the following equation is provided. ##EQU3##
According to the equation (19), the difference (IL-IR) of the currents flowing in the piezoelectric units 2 and 3 when the oscillator 1 is driven by applying the same potential on the piezoelectric units 2 and 3, is determined by the angular velocity .OMEGA.. Therefore, the angular velocity .OMEGA. is known by detecting the difference of currents (IL-IR). Further, especially, by maintaining constant an amount (IL+IR-2.times.Y.times.V) which is the sum of currents subtracted by the respective damping admittance components, the gain of the difference of currents with respect to the angular velocity .OMEGA. is determined by the equivalent mass m and the mechanical impedance ZX in the detecting-axis direction of the oscillator 1.
At this stage, an explanation will be given of a corresponding relationship between the aforementioned equations and the circuit of FIG. 25.
When the same potential V is applied on the piezoelectric units 2 and 3, the currents IL and IR flow respectively therein. As explained above, the outputs of the current-voltage converters 4 and 5 are the currents which flow when the same potential V is applied on the piezoelectric units 2 and 3 respectively connecting to the current-voltage converters 4 and 5, subtracted by the damping capacitance components of the piezoelectric units through the amplifier 10 and the condensers 11a and 11b. In this relationship, Y designates an inverse number (admittance) of the damping capacitance Cd.
Accordingly, EQU (Current-voltage converter 4): IL-Y.times.V (20) EQU (Current-voltage converter 5): IR-Y.times.V (21)
The subtraction is performed with respect to the outputs of the current-voltage converters 4 and 5 by the differential amplifier 12, and (IL-IR) is outputted from the differential amplifier 12. This is the angular velocity signal.
On the other hand, the outputs of the current-voltage converters 4 and 5 are added together by the adder 6, and the adder 6 outputs (IL+IR-2.times.Y.times.V). Further, the amplitude detecting means 8, the controller 9 and the amplifier 7 control the amplitude of the voltage V which is applied to the piezoelectric units 2 and 3 so that the amplitude of the output of the adder 6 becomes a predetermined value. Accordingly, (IL+IR-2.times.Y.times.V)=CONST (constant). The equation (19) is modified based on the above relationship, as follows. ##EQU4##
In this way, the force factors of the piezoelectric units 2 and 3 are eliminated.
Further, the phase relationship between the voltage V applied on the piezoelectric units 2 and 3 and the output (IL+IR-2.times.Y.times.V) of the adder 6 is provided by the equation (18), as follows. EQU (IL+IR-2.times.Y.times.V)/V=2.times.A.sup.2 /ZY (23)
Accordingly, by feeding back the output of the adder 6 to the piezoelectric units 2 and 3 through the amplifier 7, the oscillator 1 is oscillated by a self-excited oscillation at the resonance point of the mechanical impedance ZY in the driving-axis direction. Therefore, the phase difference between the output of the adder 6 and the applied voltage V becomes 0.degree..
Further, the phase relationship between the output of the adder 6 and the output of the differential amplifier 12 is provided by modifying the equation (19), as follows. ##EQU5##
According to the equation (24), the phase relationship between the output of the adder 6 and the output of the differential amplifier 12 is determined by the phase of an inverse number 1/ZX of the mechanical impedance in the detecting-axis direction of the oscillator 1. The resonance frequency in the detecting-axis direction is deviated from the resonance frequency in the driving-axis direction, and normally the phase difference is 90.degree..
The currents flowing in the two piezoelectric units, are superposed with the current component caused by the angular velocity and the current component caused by driving the oscillator, respectively.
The conventional oscillation gyro takes out only the current component caused by the angular velocity by calculating the difference between the currents flowing in the two piezoelectric units, as stated above.
However, the current component caused by the angular velocity is significantly small compared with the current component caused by driving the oscillator. Therefore, the former component strongly receives the influence of a common-mode rejection ratio of the differential amplifier and its S/N ratio is poor. Further, when the common-mode detection ratio of the differential amplifier changes with a change in an environmental temperature (surrounding temperature), the output of the angular velocity is influenced thereby, and especially, an angular velocity may be outputted even in a nonrotational state.
Further, the conventional oscillation gyro is constructed on the premise wherein the force factors and the damping capacitances of the two piezoelectric units are quite the same. However, actually, such a construction is difficult, and the yield of product is poor even if it is possible.
Accordingly, an angular velocity may be outputted even in a nonrotational state due to the difference in the properties of the two piezoelectric units.
Further, the difference in the properties of the piezoelectric units (force factor, damping capacitance etc.) is provided with a temperature characteristic, and therefore, the output of the angular velocity may include an error due to the change in the environmental temperature.
Further, a signal corresponding to a difference between the resistances of the piezoelectric units is caused since the properties of the piezoelectric units are not quite the same, thereby deteriorating the accuracy of the output of the angular velocity.
Further, when a frequency of change of the applied angular velocity is approximately equal to a difference between or a sum of the resonance frequency in the driving-axis direction and the resonance frequency in the detecting-axis direction, the oscillator is resonated in the detecting-axis direction, and outputs a signal which is larger than the actual angular velocity.
Further, it is not possible to inspect the characteristic of a piezoelectric unit which is employed in an oscillation gyro, in a simple way.